Merge branch 'nd/checkout-option-parsing-fix'

The option parsing of "git checkout" had error checking, dwim and
defaulting missing options, all mixed in the code, and issuing an
appropriate error message with useful context was getting harder.
Reorganize the code and allow giving a proper diagnosis when the
user says "git checkout -b -t foo bar" (e.g. "-t" is not a good name
for a branch).

* nd/checkout-option-parsing-fix:
  checkout: reorder option handling
  checkout: move more parameters to struct checkout_opts
  checkout: pass "struct checkout_opts *" as const pointer